This invention relates to a process for preparing high molecular weight polyamides, which is carried out in liquid phase, and in the absence of organic solvents or of large amounts of water.
Prior art processes for preparing high molecular weight polyamides, whether starting with a dicarboxylic acid (also called diacid) and a diamine or a diester or a diacyl chloride and a diamine, usually were carried out in solution in an organic solvent or solvent mixture, water, or a solvent-water system. The most widely practiced industrial process starts with a dicarboxylic acid and a diamine, which are combined together in a stoichiometric ratio in solution in a large amount of water, that must eventually be evaporated and removed. The solids concentration in such a solution is normally only about 50%. It can be seen that these prior art processes require large amounts of energy and, in addition, call for large capital investment because of the size and complexity of equipment, which must handle large volumes of liquids per amount of polyamide produced.
Attempts were also made in the past to make high molecular weight polyamides in the absence of solvents, but such prior art techniques were troublesome because of the difficulty in efficiently mixing and heating a partially molten reacting mass to control the reaction temperature and thus prevent local overheating. Even when good temperature control was possible, some reactants suffered thermal degradation at the high temperatures required for maintaining and handling the feed streams in liquid state. Moreover, continuous operation was difficult because of lack of adequate techniques for continuously proportioning solid starting materials in exact quantities.
An ideal process for making high molecular weight polyamides should be carried out in the liquid state because of the ease of metering and conveying the reactants, in the absence of organic solvents or large amounts of added water, and at sufficiently low temperatures to avoid substantial thermal degradation.